When a piston pushes a sheep into a hopper the sheep sinks into the piston instead of going into the hopper.
MC-144037
Mobs sink into hopper when pushed via piston
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Minecraft 19w06a, 20w30a, 1.16.2 Pre-release 2, 1.16.2 Pre-release 3, 1.16.2 Release Candidate 1, ..., 20w49a, 20w51a, 1.17, 1.17.1 Pre-release 1, 1.21.5
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I can confirm this for 20w30a. It seems when an entity like sheep is pushed by a piston into a hopper the sheep sinks into the block instead of going through the hopper.